Recently, much attention has been focused on GaN-based compound semiconductors (e.g., GaxAl1−xN, INxGa1−xN, and AlxGayIn1−x−yN, where 0≦x≦1 and y≧0.1) for blue, green, and ultraviolet light emitting diode (LED) applications. One important reason is that GaN-based LEDs have been found to exhibit efficient light emission at room temperature.
In general, GaN-based LEDs comprise a multilayer structure in which n-type and p-type GaN are stacked on a substrate (most commonly a sapphire substrate), and INxGa1−xN/GaN multiple quantum wells are sandwiched between the p-type and n-type GaN layers. A number of methods for growing the multilayer structure are known in the art, including metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).
It is also known in the art that these conventional growth methods for compound semiconductor structures, except for MBE, have proven problematic with respect to forming a p-type GaN-based layer suitable for LED applications. In general, GaN layers formed by known growth methods, such as MOCVD, and doped with p-type material such as magnesium, behave like a semi-insulating or high-resistive material. This is thought to result from hydrogen passivation caused by hydrogen that is present in the reaction chamber complexing with the p-type dopant and thus preventing the dopant from behaving as an active carrier. Because of this phenomenon, p-type GaN having a sufficiently low resistivity to form the p-n junction required for LED and laser diode (LD) applications cannot be produced by MOCVD or HVPE techniques.
Various attempts have been made to overcome the difficulties in obtaining p-type GaN-based compound semiconductors. In one technique, known as low-energy electron-beam irradiation (LEEBI), a high-resistive semi-insulating GaN layer, which is doped with a p-type impurity, such as Mg, and formed on top of multilayers of GaN-based compound semiconductors is irradiated with an electron beam having an acceleration voltage of 5 kV to 15 kV while maintaining the semiconductor material at temperatures up to 600° C., in order to reduce the resistance of the p-doped region near the sample surface. However, with this method, reduction in the resistance of the p-doped layer can be achieved only up to the point that the electron beam penetrates the sample, i.e. a very thin surface portion of less than about 0.5 μm deep. Furthermore, this method requires heating the substrate to temperatures up to approximately 600° C in addition to high-voltage acceleration of the electron beam.
Thermal annealing in a non-hydrogen atmosphere has also been used to activate the p-type dopants. For example, heat treatment at 700° C. to 800° C. in a nitrogen atmosphere is typically used to activate the Mg dopants. However, the high temperature required for activation may degrade the light-emitting efficiency of the device by damaging the lattice structure of the III-V nitride material.
In order to improve the efficiency of GaN-based semiconductor devices such as LEDs and laser diodes, it is necessary to develop improved processes for preparing p-type GaN materials which overcome or reduce these problems.